A critical step in semiconductor processing is photolithography. The ability to achieve smaller and smaller dimensions on an integrated circuit is generally understood to be limited more by photolithography than any other fabrication step of the semiconductor process. As the industry heads toward forming 0.1 micron (um) line dimensions, advanced photolithographic capabilities become ever more critical. Various new photolithographic techniques using smaller wavelength light sources are being developed; and Deep Ultra Violet regime, Extreme Ultra Violet Lithography (EUVL) also known as "soft x-ray" lithography, typically at a light wavelength of approximately 5-25 nanometers (nm), and x-ray lithography are examples of development areas. Even if commercially viable photolithography systems at the shorter wavelengths are developed, these methods, cannot be successful unless masks can be developed for them. Hence, the photolithography mask (hereafter "photomask") itself is at risk of being a technology limitation to achieving 0.1 um line widths.
The conventional method of forming a photomask starts with a reticle blank. The reticle may be made of glass or other materials such as quartz, and the material is usually transparent to the wavelength of light to be used in the photolithography system. The substrate blank is coated with an absorbing material to-be-patterned; the absorber is usually chrome, but can also be made of aluminum, tungsten, tantalum or titanium. A coating of photoresist is formed over the top of the absorbing material. The reticle is then placed in a patterning system, usually a direct electron beam or laser writing system. The writing system beam scans across the photoresist in an automated pattern, in accordance with data fed from a preprogrammed database of patterning instructions for an integrated circuit layer. After the pattern has been written into the photoresist, the photoresist is developed. Then, the absorber layer on the reticle is patterned by wet or dry etching the open areas of the photoresist created by photoresist development. Then, any remaining photoresist is removed. The reticle is inspected for defects, and defects are repaired. The reticle is then ready for use.
The pattern of the reticle is usually created in larger dimensions than the dimensions of the pattern to be made on the silicon wafer substrate because a reduction photolithography system is used, that is, one that has reducing optics. A typical reduction factor in today's photolithography systems is 4. Thus, if the desired pattern on the silicon wafer substrate has a minimum dimension of 0.4 um, then the starting reticle will be patterned at a minimum dimension of 1.6 um.
Having a starting reticle at a minimum dimension of 1.6 um is considered manufacturable today because line patterns, dimensions, defect levels and the ability to repair defects fall within today's normal manufacturing tolerances. A way to achieve smaller substrate patterns is, of course, to reduce the reticle line width. The problem with reducing reticle line width is in its manufacturability. Even if line widths are reduced on the direct write beam, the manufacturing defects seen at the larger widths are still present, creating the effect of magnified defects. Magnified defects take a freshly written reticle out of manufacturing tolerances, leading to lower reticle yields and therefor increased costs.
What is needed is a maskmaking technique that is manufacturable and useful for patterning substrates in small dimensions such as 0.1 um. It would be advantageous to be able to use presently characterized and understood fabrication techniques in maskmaking.